A guess from practical experience with horse tack and bags ranging from use to repair and construction?
Rivets are more expensive than thread and they were probably harder to get in remote locations. They also provide more strength than needed. From the commercial standpoint there was also probably a much bigger demand for horse tack repair and manufacture than for bags. Other than a quick repair or perhaps a nod to decoration, they don't seem like they would be even a little bit practical.
Take that as a measure of my outlook that practicality trumped most other considerations on the various frontiers.
